I have two different domains, a forum and a blog, both about books and reviews. Tha last one is quite new, just about 2-3 months old. In the last weeks I've published several equal book reviews on both websites. For example there are two identical articles about Pride and Prejudice and the one on the forum has a link pointing to the one on the blog, saying something like "hey, this review has been published also in our blog, you can read it here". Well, today I saw on Tools for Webmasters that both have been probably penalized since impressions on SERPS suddenly dropped down of about 50% on both sites and in the same day (6th February). Now I got rid immediately of reviews on the forum, keeping only the ones on the blog but I'm not sure that it will be enough to make Google reconsider both domains. What should I do and how much time will it take to recover?
Do nothing and wait. If the duplicate content was really the issue then this will resolve it and your rankings will return in time. There is no timetable for this so we won't be able to give you one. But if you start to change other things then it becomes impossible to tell if this was really the issue or not and then correcting it becomes much more difficult.